1. Field of the Invention
This invention relates to the centrifugal separation of a particle laden gas into a particle-free stream and a particle-rich stream.
2. Discussion of Background
Several investigators have proposed methods for achieving the separation of particles from a gas stream by use of centrifugal force. One of the first was Braun (U.S. Pat. No. 165,785), who proposed the use of a rotating drum and internal vanes. In this design, particles are flung against the inner wall of the rotating drum and deposited. Once deposited, the mass of particles continues to move under the influence of the centrifugal force to a central gutter, from which it can be removed by means of a valve.
In the approach used by Lincoln (U.S. Pat. No. 431,336), several flat disks are placed parallel to each other on a hollow hub. Each disk is fitted with several scoop-like attachments. While the disks rotate about the hub, a gas flow is introduced parallel to the disks. The scoop-like attachments on the disks serve as particle impaction surfaces where particles are deposited. By the centrifugal influence of the rotating assembly, the particles are swept into a channel that leads to the hollow hub. Once inside the hollow hub, the particles drop into a hopper for later removal.
The operating principle of Sakowski's apparatus (U.S. Pat. No, 2,849,078) is very similar to that of Braun. Cylindrical pipes are formed into semicircles. Four semicircles are joined so as to form two circles, the planes of which are at right angles to each other. A particle bearing gas is introduced at the juncture of the four semicircles and flows through the pipes to an exit manifold at the other juncture of the pipes. At the apex of each semicircle (midway between the inlet and exit manifolds) is an exit tube connected to another circle-shaped pipe. The entire apparatus is mounted in a rotatable fixture. The rotational motion of the apparatus produces a centrifugal force on the particles in the gas and causes them to impact against the surfaces of the pipes. The collected particle mass is emitted through the exit tubes into the outermost pipe circle, from which it is dropped into a stationary gutter for later removal.
The invention of Bouru (U.S. Pat. No. 3,561,195) was designed for the removal of oil droplets from a gas stream, but also has application to removal of solid particles. This device relies upon a cylinder fitted with a large number of vanes and mounted coaxially with a pipe for evacuation of the purified gas. In the design, an annular inlet space is formed between the outer cylinder and the coaxial pipe, and the entire assembly is rotatably mounted. The rotational motion of the assembly imparts a centrifugal force on the particles in the entering gas stream. The particles are then impacted on the vanes and inner surfaces of the outer cylinder. Centrifugal force on the collected particles causes them to migrate to the outermost edge on the interior of the cylinder. The cleaned gas is redirected towards the evacuation pipe while the collected particles are emitted from the outer cylinder by means of small holes in the periphery. While it is clear that collected oil droplets would easily flow through these openings, successful operation with solid particles appears uncertain.
Peterson et al (U.S. Pat. No. 4,134,744) also use a rotating disk in a particle collection device. In their approach, the disk is composed of electrically isolated wedges that are oppositely charged. A dielectric fluid is continually pumped onto the surface of the disk where it is dispersed by the centrifugal force of the rotation. Charged particles are attracted to the polarized surfaces of the disk where they impact with the dielectric fluid, whereupon they become fixed in the fluid. As the fluid is centrifugally pumped to the edge of the disk, the particles are carried with it, eventually to be flung against a toroidal ring that serves as a gutter for fluid removal. In this device the particles are collected by electrostatic forces and then fixed in a solution. The centrifugal action simply serves as a means for continually renewing the solution surface on the disk.
Another centrifugal-based device is that of Alink et al (U.S. Pat. No. 4,512,759) in which a gas stream is introduced into a diverging cone that rotates. This device is similar to that of Braun, in that the rotating vessel imparts a rotational flow to the particles in the gas stream. As a result of centrifugal force the particles are flung against the wall of the rotating cone where they remain deposited. The collected mass of particles moves to the edge of the diverging cone where it is deposited in a concentric duct and falls by gravity into a hopper for later removal.
The device of Andersson (U.S. Pat. No. 4,530,462) separates pneumatically conveyed streams of solid or liquid particles into a number of streams containing equal concentrations of particles. Particles are carried by centrifugal force resulting from a rotational flow induced by means of a series of helical flow channels. The particles are flung against a collecting surface and migrate to a buffer area. From the buffer, the particles are either deposited uniformly around the circumference of a collection plate or are dropped into hopper regions to be reentrained with the redistributed gas flow.
All of the devices described above rely upon impaction of the dust against a surface to achieve separation from the gas stream. Several other devices have been developed for the separation of particles from a gas stream. The invention of Mercier et al and Ehlinger (U.S. Pat. No. 2,603,309) employs a rapid change of flow direction for gas at high velocities. In this device, a gas stream exits a narrow channel between parallel plates at high velocity. Inertial force carries particles towards a hopper region where they may impact on surfaces or settle under the influence of gravity. The gas stream is evacuated through a narrow channel parallel and immediately adjacent to the inlet channel. Lefkowitz (U.S. Pat. No. 4,473,384) extended this approach by introducing an impaction stage at the exit of an accelerator nozzle. In this approach, particles are impacted on the surfaces of circular bands, which are attached to a rotating drum. The circular impaction bands are swept clean by a brush-type apparatus as the drum rotates. As before, the gas experiences a rapid change of flow direction and is evacuated through the hollow hub of the rotating drum.
Another particle separation device is that of Tillman et al. (U.S. Pat. No. 3,252,275), which is intended for the collection of agglomerating particles (especially titanium dioxide). This device consists of a drum with a rotating paddle on the inside. The rotating paddle enhances the agglomeration of particles. As the particles grow, they settle by gravity to the bottom of the chamber where they are removed to a hopper. This device would be of limited utility in the case of a non-agglomerating aerosol.